I'm trying to set 'Wake the Computer To Run This Task' as default true but I can't figure out how.

Even when going though the demo, just trying to change that option doesn't seem to work. If I try to just change the wake computer option, it doesn't change (after leaving the task properties and getting back in).

Any ideas?

I've done a quick check of the previous posts and haven't come across this question, so hopefully I'm not duplicating anything.

You need to include tfSystemRequired into Flags property of TTaskItem. If you run the demo application you can set this option using either Advanced button in New Task Wizard or by activating System Required option on Settings Tab when clicking Properties for selected task.
Please let me know if this helps.